Wdm type passive optical network

ABSTRACT

In a WDM type PON system, each ONU comprises an optical transmitter capable to transmit optical signals with variable wavelengths, an optical signal receiving filter variable its receiving wavelength, and a control unit. An OLT selects in response to a wavelength allocation request from each ONU, a transmitting wavelength and a receiving wavelength out of currently free wavelengths and allocates these wavelengths to the requester ONT. The control unit of the ONU switches the transmitting wavelength of the optical transmitter and the receiving wavelength of the optical signal receiving filter to the wavelengths specified in a response message from the OLT and starts data communication.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial No. 2005-222483, filed on Aug. 8, 2005, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a Passive Optical Network (PON) inwhich a plurality of subscriber connection apparatuses share a part ofan optical transmission line and, more particularly, to a WDM type PONsystem utilizing an Optical Wavelength Division Multiplexing (WDM)technique for multiplexing a plurality of optical signals with differentwavelengths in the PON.

(2) Description of Related Art

A Passive Optical Network (PON) system is known as one of access networksystems for point to multi-points (one to N) connection using opticalsignals. The PON system comprises a central office side apparatus OLT(Optical Line Terminal) placed at a facility center operated by anetwork vendor such as a type 1 carrier and a plurality of subscriberconnection apparatuses ONUs (Optical Network Units) placed at userhomes, respectively. An optical fiber connected to the OLT is divergedby a splitter (optical branching filter) into a plurality of branchoptical fibers on the order of 8 to 64 lines. Each ONU is connected toone of branch optical fibers. The PON system enables significantreduction in the cost for optical fiber network construction because aplurality of subscribers can share the optical fiber section between thesplitter and the OLT.

A wavelength division multiplexing type PON (WDM-PON) applies thewavelength multiplexing technique in the shared fiber section of the PONand following two schemes are known:

(a) A first scheme allocates different wavelengths to upstream anddownstream signals and a plurality of ONUs use the upstream wavelengthλu and the downstream wavelength λd in a time division multiplexing(TDM) manner. In this case, an optical power splitter operable withoutpower supply is used as the splitter and each ONU transmits and receivesdata within a time slot allocated from the OLT.

(b) A second scheme allocates dedicated upstream signal wavelength anddownstream signal wavelength to each ONU. In this case, the splitterneeds a filter function to separate a downstream signal transmitted in awavelength multiplexing manner from the OLT into individual wavelengthsand to distribute them to respective ONUs. According to the secondscheme, each ONU can get data addressed to it by receiving only thewavelength allocated to it beforehand.

The first scheme allows the use of optical transceivers that are commonfor all ONUs because there is only a difference between the upstreamsignal wavelength and the downstream signal wavelength. Further, thisscheme is regarded as economical, since the optical power splitter isapplicable in the shared fiber section of the PON. Considering that awider band would be required, in the future to transmit a large quantityof data over the PON, the second scheme improvable the transmissionspeed by wavelength multiplexing attracts attention. However, becauseeach ONU has to use a specific wavelength different from that for otherONUs, the diversity of the ONUs increases in the first scheme, as thenumber of wavelengths to be used in the PON system increases.Consequently, this scheme increases the cost of the ONUs much greaterthan in the first scheme. In other words, the second scheme requirespreparation of more diversified ONUs because all ONUs connected to theOLT transmit and receive optical signals at different wavelengths,respectively. In order to accommodate a new subscriber terminal to theOLT, it is necessary for the second scheme to prepare an ONU that hasupstream/downstream wavelengths other than the wavelengths for theexisting ONUs and to connect the ONU to an appropriate port of thesplitter/filter and, thus, is not easy to add a new ONU.

As a prior art relating to the above ONU diversification problem in theWDM-PON system, for example, Japanese Unexamined Patent Publication No.2004-222255 (patent document 1) proposes the application of spectrumslicing to share an upstream signal wavelength. In patent document 1, awide-band light with a wide spectrum is used as an upstream signal to betransmitted from each ONU. Wide-band upstream optical signals areconverted into narrow-band optical signals corresponding to ONUs byspectrally slicing the wide-band optical signals that are incoming froma plurality of branch optical fibers by a wavelength splitter, therebyto transmit the narrow-band optical signals to the OLT.

FIG. 9 shows a WDM-PON system setup proposed in patent document 1.

OLT 10 is connected via a wavelength splitter 20 to a plurality of ONUs30 (30-1 to 30-n). The OLT multiplexes downstream optical signals havingindividual wavelengths (λd1, λd2, . . . , λdn) allocated to theplurality of ONUs 30 so that an wavelength multiplexed optical signal(λd1+λd2+ . . . +λdn) is transmitted to an optical fiber transmissionpath 50.

The wavelength multiplexed optical signal is separated into theindividual wavelengths by the wavelength splitter 20 and branched awayto the branch optical fibers as downstream optical signals withdifferent wavelengths λd1, λd2, . . . , λdn for each ONU. On the otherhand, the ONUs 30-1 to 30-n transmit upstream optical signals at thesame wavelength λu for all the ONUs.

The wavelength splitter 20 spectrally slices the optical signalsincoming from the branch optical fibers into upstream optical signalshaving different wavelengths λu1, λu2, . . . , λun for each ONU, wherebythe incoming signals are converted into a wavelength multiplexed opticalsignal (λu1+λu2+ . . . +λun), and transmitted to the optical fibertransmission path 50.

FIG. 10 illustrates a relationship between the upstream signalwavelengths and the downstream signal wavelengths used in the WDM-PONsystem of the above patent document 1.

Out of a wavelength grid in which wavelengths are sequenced at an equalcenter-to-center distance of Δλd, an arbitrary wavelength % u isselected as the upstream signal wavelength and the remaining wavelengthsλd1, λd2 through λdn are used as the downstream wavelengths. Thedownstream signal wavelengths have a wavelength error within Δλd/2. Thewavelength λu is spectrally sliced into multiple wavelengths λu1 to λuncorresponding to the ONUs 30-1 to 30-n, respectively. The spectrallysliced upstream signals are sequenced at an equal center-to-centerdistance of Δλu and have a wavelength error within ±Δλu/2.

In order to accommodate all channels of the upstream optical signalswithin a bandwidth corresponding to one channel of downstream opticalsignal, according to patent document 1, the center-to-center distance ofΔλd for the wavelengths of the downstream optical signals is defined tobe two times or more as much as the center-to-center distance of Δλu forthe wavelengths of the upstream optical signals and the center-to-centerdistance for the wavelengths λu1 to λun of the upstream optical signalsis narrowed sufficiently. In patent document 1, it is suggested that anoptical filter for wavelength selection is removably installed in theoptical transceiver unit of each ONU in order to receive a downstreamsignal at different wavelength for each ONU.

As a prior art that aimed at effective use of wavelength in the PONsystem, for example, Japanese Unexamined Patent Publication No. Hei10-247896 (patent document 2) proposes a PON system in which each ONUdynamically switches a wavelength and a time slot for receiving adownstream signal in response to an instruction from the OLT.

FIG. 11 shows a WDM-PON system setup proposed in patent document 2.

OLT 10 is connected via a wavelength splitter 21 to a plurality of ONUs30 (30-1 to 30-n). Each ONU 30 transmits upstream signal in a time slotdesignated from the OLT, using an optical signal at the same wavelengthλx. The upstream signals transmitted from each ONU 30 are time divisionmultiplexed (TDM) on an optical fiber 50 and arrive at the OLT. On theother hand, downstream signals are transmitted from the OLT 10 to theoptical fiber 50 in a form of a wavelength multiplexed (WDM) signal withwavelengths λ1 and λ2, and branched to each ONU 30 through the opticalsplitter 21.

FIG. 12 illustrates a method of allocating to each ONU a receivingwavelength and a time slot for receiving downstream signals, proposed inpatent document 2.

The OLT 10 transmits a frame A (TF1, λ1) and a frame C (TF2, λ1), eachcomprising an over header OH and four time slots, using the wavelengthλ1, as shown in S1. In parallel with this, the OLT 10 transmits a frameB (TF1, λ2) and a frame D (TF2, λ2), each comprising an over header OHand four time slots T1 to T4, using the wavelength of λ2, as shown inS2. In the OH of each frame, as bandwidth allocation information,information designating ONUs to which the time slots T1 to T4 in theframe of the wavelength λ1 and the time slots T1 to T4 in the frame ofthe wavelength λ2 are allocated is set.

In the example shown here, the bandwidth allocation information “(λ2)ONU2355” for the frame A indicates that the time slots T1, T2, T3, T4 inthe frame B which is transmitted at the wavelength λ2 in parallel withthe frame A are allocated to ONU-2, ONU-3, ONU-5, ONU-5, respectively.The information “(λ1)ONU1144” indicates that the time slots T1, T2, T3,T4 in the frame A which is transmitted at the wavelength λ1 areallocated to ONU-1, ONU-1, ONU-4, ONU-4, respectively. In the OH of theframe B, the bandwidth allocation information corresponding to that forthe frame A is set. For the subsequent frames C and D as well, ONUs towhich the time slots T1, T2, T3, T4 are allocated are specified for eachwavelength.

Each of the ONUs is provided with tunable optical filter and receivesdownstream signals from the OLT at the wavelength λ1 or λ2, selectively.Whether the current receiving wavelength is λ1 or λ2, each ONU can knowits allocated time slot during receiving the OH. Accordingly, each ONUcan receive downstream signals in the allocated time slot byappropriately switching to the receiving wavelength as instructed fromthe OLT. In the example shown, the ONU-4 should perform receivingoperation in the third and fourth time slots of the frame A (TF1, λ1),in the first and second time slots of the frame D (TF2, λ2), and in thethird and fourth time slots of the frame C (TF2, λ1).

As stated above, according to the PON system of patent document 1,selection of an ONU having an unused wavelength or a filter replacementwork is required at the time of ONU installation, since an individualwavelength must be fixedly allocated to each ONU for receivingdownstream optical signals. Further, this type of PON system involvesincreased management cost because it requires to record wavelengthscurrently used in preparation for increasing ONUs later.

On the other hand, the PON system of patent document 2 adopts timedivision multiplexing at the same wavelength for upstream signals. Inprinciple, it also adopts time division multiplexing for downstreamsignals, and the number of time slots that can be allocated to each ONUis increased simply by increasing the number of wavelengths applied fordownstream signals. Therefore, the PON system of patent document 2 is amodification example to the above-mentioned first scheme, but not a PONsystem of the second scheme.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a WDM type PON systemthat has made increasing ONUs easier.

Another object of the present invention is to provide a WDM type PONsystem in which an optical power splitter is applicable to an opticalfiber section of the PON.

Further object of the present invention is to provide a WDM type PONsystem that has made it possible to make effective use of communicationresources in the optical fiber section and widen the communicationbandwidth for each ONU by dynamically allocating to each ONU a dedicatedwavelength different from the wavelengths allocated to other ONUs, forboth upstream and downstream signals.

In order to achieve the foregoing objects, in a WDM type PON system ofthe present invention, each subscriber connection apparatus ONUtransmits and receives data, using a individual wavelength specifiedfrom a central office side apparatus OLT. The passive optical networkcomprises an optical fiber connected to the OLT, a plurality of branchoptical fibers each connected to one of the ONUs, and an optical powersplitter for distributing to the branch optical fibers a wavelengthmultiplexed downward optical signal transmitted from the OLT andcombining upstream optical signals transmitted from the ONUs into awavelength multiplexed signal which is transferred along the opticalfiber to the OLT. The optical signals transmitted from the plurality ofONUs are input to the OLT as a wavelength multiplexed signal. To eachONU, a downstream wavelength multiplexed optical signal transmitted fromthe OLT is input.

One feature of the present invention resides in that the OLT selects atransmitting wavelength and a receiving wavelength out of currently idlewavelengths in response to a wavelength allocation request from eachONU, and allocates these wavelengths to the requester ONT.

Another feature of the present invention resides in that each ONUcomprises an optical transmitter variable its transmitting wavelength,an optical signal receiving filter variable its receiving wavelength,and a control unit for controlling the transmitting wavelength of theoptical transmitter and the receiving wavelength of the optical signalreceiving filter, wherein the control unit issues a wavelengthallocation request message to the OLT, switches the transmittingwavelength of the optical transmitter and the receiving wavelength ofthe optical signal receiving filter to specific wavelengths designatedin a response message from the OLT, and starts data transmission andreception.

More specifically, in the WDM type PON system of the present invention,the central office side apparatus OLT comprises a wavelength managementtable having a plurality of entries each indicating an identifier of awavelength available in the PON and a current state of the wavelength,and a control unit for searching the wavelength management table for anidentifier of an idle wavelength in response to a wavelength allocationrequest from each ONU and allocating transmitting and receivingwavelengths corresponding to the wavelength identifier to the requesterONU. The control unit of each ONU transmits a wavelength allocationrequest to the OLT when the transmitting wavelength of the opticaltransmitter and the receiving wavelength of the optical signal receivingfilter are at wavelengths that are common for all ONUs, and switches thetransmitting wavelength of the optical transmitter and the receivingwavelength of the optical signal receiving filter to specificwavelengths allocated from the OLT when a wavelength allocation responsemessage was received from the OLT.

In the WDM type PON system of the present invention, the OLT comprises,for example, a common interface unit for transmitting and receivingoptical signals at wavelengths that are common for all ONUs, a pluralityof individual interface units for transmitting and receiving opticalsignals at wavelengths corresponding to the wavelength identifiersregistered in the wavelength management table, and an optical unit forseparating a wavelength multiplexed optical signal incoming from theoptical fiber into individual wavelength optical signals to selectivelydistribute the individual wavelength optical signals to the interfaceunits corresponding to the wavelengths, and wavelength multiplexing theoptical signals output from the interface units to transmit as awavelength multiplexed optical signal to the optical fiber. The controlunit of the OLT communicates control messages for wavelength allocationwith each ONU through the common interface unit.

According to the present invention, when a new ONU is added to the PON,the engineer involved does not need to care for wavelengths to be usedby the additional ONU, since each ONU is able to autonomously start datatransmission and reception at individual transmitting and receivingwavelengths specified from the OLT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural diagram showing an embodiment of a WDM-PON systemaccording to the present invention.

FIG. 2 illustrates a relationship between upstream optical signalwavelengths and downstream optical signal wavelengths used in the firstembodiment of the invention.

FIG. 3 is a structural diagram showing an embodiment of a subscriberconnection apparatus (ONU) 30 shown in FIG. 1.

FIG. 4 is a structural diagram showing an embodiment of a central officeside apparatus (OLT) 10 shown in FIG. 1.

FIG. 5 exemplifies an embodiment of a wavelength management table 120provided in a wavelength negotiator 300 of the central office sideapparatus (OLT) 10.

FIG. 6 is a sequence diagram of wavelength negotiation to be carried outin the PON system of the present invention.

FIG. 7 shows an example of a frame format of control messages to becommunicated during wavelength negotiation.

FIG. 8 is a sequence diagram of a health check to be carried outperiodically by the OLT 10.

FIG. 9 shows one example of a PON system setup of prior art.

FIG. 10 illustrates a relationship between the upstream signalwavelengths and the downstream signal wavelengths used in the PON systemof FIG. 9.

FIG. 11 shows another example of a PON system setup of prior art.

FIG. 12 illustrates a method of allocating receiving wavelengths andtime slots for down signals in the PON system of FIG. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinafter with reference to the drawings.

FIG. 1 is a structural diagram showing an embodiment of a WDM-PON systemaccording to the present invention.

The PON system of the present invention comprises an central office sideapparatus (OLT) 10, a plurality of subscriber connection apparatuses(ONUs) 30 (30-1 to 30-n), and a passive optical network in which anoptical fiber 50 and a plurality of branch optical fibers 51 (51-1 to51-n) are linked via an optical power splitter 53.

Each ONU 30-j (j=1 to n) is equipped with a modulation type opticaltransmitter 313 whose wavelength is variable and transmits an upstreamoptical signal to a branch optical fiber 51-j at a specific wavelengthλuj (j=1 to n) allocated from the OLT. The upstream optical signaltransmitted form each ONU is multiplexed together with upstream opticalsignals transmitted from other ONUs by the optical power splitter 53 andtransmitted to the optical fiber 50 as a wavelength multiplexed signal.

The upstream optical signal of multiple wavelengths λu1 +λu2+ . . . +λunis wavelength demultiplexed by an Arrayed Waveguide Grating (AWG) 11 inthe OLT 10 and the demultiplexed signals are selectively distributed tointerface (INF) units 12 (12-1 to 12-n) corresponding to thewavelengths. In the example shown here, an optical signal at awavelength λu1 transmitted from an ONU 30-1 is supplied to an INF unit12-1, an optical signal at a wavelength λu2 transmitted from an ONU 30-2is supplied to an INF unit 12-2, and an optical signal at a wavelengthλun transmitted from an ONU 30-n is supplied to an INF unit 12-n,respectively.

Here, each of the INF units 12-1 to 12-n converts the upstream opticalsignal into an electrical signal, performs PON termination processing,and outputs the signal as an upstream packet. The upstream packet outputfrom each INF unit is input to a higher-level apparatus (not shown) viaa switching unit 13 and, after subjected to protocol conversion, ifnecessary, transferred onto a higher-level network such as the Internet.

On the other hand, packets received from the higher-level network aredistributed to the INF units 12-1 to 12-n by the switching unit 13 and,after converted into PON frames, output as optical signals to the AWG11. In the example shown here, the INF unit 12-1 outputs a downstreamoptical signal at a wavelength λd1, the INF unit 12-2 outputs adownstream optical signal at a wavelength λd2, and the INF unit 12-noutputs a downstream optical signal at a wavelength λdn to the AWG 11.

The AWG 11 multiplexes these downstream optical signals by wavelengthmultiplexing and transmits as a downstream optical signal of multiplewavelengths λd1+λd2+ . . . +λdn to the optical fiber 50. The wavelengthmultiplexed downstream optical signal passes through the splitter 53without being wavelength demultiplexed and supplied to the ONUs 30-1 to30-n. Each ONU 30-j (j=1ton) is equipped with a variable wavelengthfilter 320 at a pre-stage of an optical receiver 321. The variablewavelength filter 320 selectively receives only an optical signal at aspecific wavelength λdj allocated from the OLT 10 and outputs it to theoptical receiver 321. Here, the variable wavelength filter 320 of theONU 30-1 selectively receives an optical signal at a wavelength λd1, thevariable wavelength filter 320 of the ONU 30-2 selectively receives anoptical signal at a wavelength λd2, and the variable wavelength filter320 of the ONU 30-n selectively receives an optical signal at awavelength λdn.

A feature of the present invention resides in that the OLT 10 and eachONU 30 j are equipped with wavelength negotiators 100 and 300,respectively, and that the wavelength negotiator 100 of the OLT 10allocates, in response to a wavelength allocation request from thewavelength negotiator 300 of the ONU 30 j, an upstream optical signalwavelength λuj and a downstream optical signal wavelength λdj to be usedby the ONU 30 j.

The wavelength allocation request is transmitted, using a wavelength λucthat is common for all ONUs for control information transmission. Anupstream optical signal at the wavelength λuc is distributed to a commonINF unit 12-0 by the AWG 11 in the OLT 10. The common INF unit 12-0converts the optical signal at the wavelength λuc into an electricalsignal, performs PON termination processing, and outputs the signal as acontrol packet to the wavelength negotiator 100. A response packet forwavelength allocation, generated by the wavelength negotiator 100, isconverted into a downstream optical signal at a wavelength λuc that iscommon for all ONUs by the common INF unit 12-0. The signal of thewavelength λuc is multiplexed with downstream optical signals at otherwavelengths (λd1+λd2+ . . . +λdn) by wavelength multiplexing at the AWG11 and transmitted to the optical fiber 50.

The wavelength negotiator 300 of the ONU 30 j sets the receivingwavelength of the variable wavelength filter 320 to the commonwavelength λdc, as long as the transmitting wavelength of the modulationtype optical transmitter 313 is set to the common wavelength % uc. Whenwavelengths λuj and λdj to be used are specified by a wavelengthallocation response packet from the OLT 10, the wavelength negotiator300 switches the transmitting wavelength of the modulation type opticaltransmitter 313 from the common wavelength λuc to the dedicatedwavelength λuj and switches the receiving wavelength of the variablewavelength filter 320 from the common wavelength λdc to the dedicatedwave length λdj by using a control signal Sd.

FIG. 2 illustrates a relationship between upstream optical signalwavelengths and downstream optical signal wavelengths to be used in thepresent embodiment.

Wavelengths (λdc, λuc) that are common for all ONUs for controlinformation transmission are prepared along with upstream optical signalwavelengths (λu1, λu2, . . . λun) and downstream optical signalwavelengths (λd1, λd2, . . . λdn) to be allocated to each ONU 30 j. Whenissuing a wavelength allocation request, the wavelength negotiator 300of each ONU 30 j executes negotiation with the OLT 10 in a state wherethe transmitting wavelength of the modulation type optical transmitter313 is set to λuc and the receiving wavelength of the variablewavelength filter 320 is set to λdc.

FIG. 3 shows an embodiment of a subscriber connection apparatus (ONU)30-1. Other ONUs 30-2 to 30-n are configured in the same way as shownhere.

The ONU 30-1 comprises a wavelength negotiator 300, a terminal interface304 for connection to a subscriber terminal, an optical fiber connectionunit 305 for connection to a branch optical fiber 51-1, and an upstreamsignal processing circuit and a downstream signal processing circuitprovided between the terminal interface 304 and the optical fiberconnection unit 305.

The upstream signal processing circuit comprises an upstream data buffer311 for temporarily storing transmission data from a subscriberterminal, received through the terminal interface 304, an upstream framegenerator 312 which edits transmission data read out from the upstreamdata buffer 311 into PON upstream frames (packets), and a modulationtype optical transmitter (E/O converter) variable in transmittingwavelength, which converts upstream data output from the upstream framegenerator 312 into an optical signal at a specific wavelength λu1 andoutputs the optical signal to the optical fiber connection unit 305.

On the other hand, the downstream signal processing circuit comprises avariable wavelength filter 320 for selectively receiving an opticalsignal at a specific wavelength λd1 out of a wavelength multiplexedoptical signal output from the optical fiber connection unit 305, anoptical receiver (O/E converter) 321 for converting an optical signaloutput from the variable wavelength filter 320 into an electricalsignal, a downstream frame termination unit (PON termination unit) 322connected to the optical receiver 321, and a downstream data buffer 323for temporarily storing received data output from the downstream frametermination unit (PON termination unit) 322 and outputting it to theterminal interface 304. The downstream frame termination unit 322performs termination processing of PON downstream signals and analyzesthe received frames (packets). If a control frame is received, thetermination unit outputs the received frame to the wavelength negotiator300. If a user frame is received, the termination unit outputs thereceived frame to the downstream data buffer 323.

In the present embodiment, the wavelength negotiator 300 comprises aprocessor 301, a main memory 302 for storing a wavelength negotiationprogram, a nonvolatile memory 303, and an internal bus 304 forinterconnecting these components. In the main memory 302, variousroutines for implementing the functions of the ONU is stored besides thewavelength negotiation program. The processor 301 functions as awavelength negotiator during the execution of the wavelength negotiationprogram and functions as the control unit of the ONU during theexecution of any other program.

The nonvolatile memory 303 stores the wavelength negotiation program andother programs. These programs are copied into the main memory 302 bythe processor 301 when the ONU is activated. The setting of thetransmitting wavelength λu1 for the modulation type optical transmitter313 and the setting of the receiving wavelength λd1 for the variablewavelength filter 320 are carried out by the processor 301 via theinternal bus 304 and signal lines Su, Sd.

FIG. 4 shows an embodiment of the central office side apparatus (OLT)10.

The OLT 10 comprises an AWG 11, a wavelength negotiator 100, a switchingunit 13, a common INF unit 12-0 for control frames, connected betweenthe AWG 11 and the wavelength negotiator 100, and a plurality ofindividual INF units 12-1 to 12-n connected between the AWG 11 and theswitching unit 13. In the present embodiment, the wavelength negotiator100 comprises a processor 101, a main memory 102 for storing awavelength negotiation program 110 and a wavelength management table120, a nonvolatile memory 103, and an internal bus 104 forinterconnecting these components.

In the main memory 102, various routines for implementing the functionsof the OLT is stored besides the wavelength negotiation program. Theprocessor 101 functions as a wavelength negotiator during the executionof the wavelength negotiation program and functions as the control unitof the OLT during the execution of any other program. The nonvolatilememory 103 stores the wavelength negotiation program and other programsas well as the wavelength management table 120 and other tables andthese programs and tables are copied into the main memory 102 by theprocessor 101 when the OLT is activated.

The processor 101 carries out wavelength negotiation with each ONU viathe common INF unit 12-0. Upon allocating wavelengths λuj, λdj to an ONU30-j, the processor 101 activates an individual INF unit 12-j for theallocated wavelengths λuj, λdj by using a control signal Se. Further,the processor 101 periodically monitors the operating state of the ONU30-j to which the wavelengths λuj, λdj were allocated. When the responsefrom the ONU 30-j ceased, the processor 101 stops the operation of therelevant individual INF unit 12-j.

FIG. 5 exemplifies an embodiment of the wavelength management table 120to be used by the wavelength negotiator 100 of the OLT 10 to manage thestates of wavelength allocations to the ONUs.

The wavelength management table 120 comprises a plurality of entries120-1 to 120-m each indicating relationship between a wavelength index121 (wavelength identifier) and use state 122 of the wavelength. The usestate 122 of the wavelength indicates one of the following three states:“idle (free),” “under adjustment,” and “under use.”

FIG. 6 shows a sequence diagram of wavelength negotiation to be carriedout in the PON system of the present invention. By way of example, asequence starting with an available wavelength allocation request issuedfrom an ONU-1 (30-1) to the OLT 10 will be described here.

When the ONU-1 (30-1) is powered on, the processor 301 sets thetransmitting wavelength of the modulation type optical transmitter 313to the common wavelength % uc and the receiving wavelength of thevariable wavelength filter 320 to the common wavelength λdc (400) andestablishes a session with the OLT 10 (401). This session isestablished, for example, by carrying out a connection sequence in theEPON Discovery process in compliance with IEEE 802.3ah. When the sessionwith the OLT 10 has been established, the processor 301 of the ONU 30-1transmits a wavelength allocation request message M1 to the OLT 10,using the common wavelength λuc.

Upon receiving the wavelength allocation request message M1 from the ONU30-1, the processor 101 of the OLT 10 executes the wavelengthnegotiation program. During wavelength negotiation with one ONU, theprocessor 101 places the wavelength management table 120 in an exclusivecontrol state to inhibit access to the wavelength management table 120by a wavelength allocation request occurring subsequently and searchesthe wavelength management table 120 for an idle wavelength λx (402).This searching for an idle wavelength λx means to search for a tableentry 120-k in which the use state 122 is “idle (free)” from thewavelength management table 120. This search for an idle wavelength λx(402) is sure to succeed, because the wavelength management table 120has a sufficient number of table entries for allowing wavelengthallocation to all ONUs under the control of the OLT 10.

When an idle wavelength λx was found, the processor 101 changes the usestate 122 in the table entry 120-k from “idle” to “under adjustment”(403) and releases the wavelength management table 120 from theexclusive control. After that, the processor 101 generates a responsemessage M2 including identifiers of allocated wavelengths (λux and λdx)corresponding to the value of the wavelength index 121 in the tableentry 120-k and transmits the response message M2 to the ONU 30-1 at thecommon wavelength λdc through the INF unit 12-0 (404). Here, thewavelengths λux and λdx to be allocated to the ONU 30-1 are arbitraryones of idle wavelengths and the OLT 10 does not allocate the samewavelengths statically, every time in response to a wavelengthallocation request from the ONU 30-1.

Upon receiving the wavelength allocation response message M2, thewavelength negotiator 300 (processor 301) of the ONU 30-1 transmits anwavelength allocation acknowledgement message M3 to the OLT 10 at thecommon wavelength λuc (405). Then, the processor 301 changes thetransmitting wavelength of the modulation type optical transmitter 313from the common wavelength λuc to the allocated wavelength λux andchanges the receiving wavelength of the variable wavelength filter 320from the common wavelength λdc to the allocated wavelength λdx (406). Ifit is necessary to change the characteristics of the optical receiver321 in accordance with the receiving wavelength of the variablewavelength filter 320, the processor 301 changes the characteristics ofthe optical receiver 321 in association with the change of thewavelength of the variable wavelength filter 320.

Upon receiving the wavelength allocation acknowledgement message M3 fromthe ONU 30-1, the processor 101 of the OLT 10 places the wavelengthmanagement table 120 in the exclusive control state again, changes theuse state 122 in the table entry 120-k from “under adjustment” to “underuse” (407), and releases the wavelength management table 120 from theexclusive control. After that, the processor 101 activates the INF unit12-x corresponding to the wavelength index “x” of the table entry 120-k(408).

FIG. 7 shows an example of a frame format of control messages to becommunicated between the ONU 30 and the OLT 10 in the above-describedwavelength negotiation. The format shown here is based on the EPON frameformat in compliance with IEEE 802.3ah.

The frame including a control message comprises a preamble 50, adestination address 51, a source address 52, a Type/Length field 53 forspecifying the frame type at the MAC layer and data length, a data part54, and a Frame Check Sequence (FCS) 55 to be used for error check ofthe frame.

The preamble comprises a reserved field 501, a Start of Packet Delimiter(SPD) 502, a reserved field 503, a Logical Link Identifier (LLID) 504,and a Cyclic Redundancy Check (CRC) 505. If the data part 54 includes awavelength allocation request message M1 or a wavelength allocationacknowledgement message M3, the MAC address of the OLT 10 is set as thedestination address 51 and the MAC address of the requester ONU is setas the source address 52. If the data part 54 includes a wavelengthallocation response message M2, the MAC address of the requester ONU isset as the destination address 51 and the MAC address of the OLT 10 isset as the source address 52.

The data part 54 includes data particular to a control message forwavelength negotiation. As a frame type 541, specific code correspondingto the type of a control message M1, M2 or M3 is set. In a field ofallocated upstream wavelength 542 and a filed of allocated downstreamwavelength 543, the identifier of a wavelength λux for upstream opticalsignals and the identifier of a wavelength % dx for downstream opticalsignals, allocated to the ONU 30-1 by the OLT 10, are set. In the caseof a wavelength request message M1, the wavelength fields 542, 543 areempty.

When the OLT 10 transmits the wavelength allocation response message M2to the ONU 30-1, ONUs that are already operating do not receive thisresponse message M2 because their receiving wavelengths are set atindividual wavelengths other than the common wavelength λdc. If anotherONU other than the ONU 30-1 has issued a wavelength allocation requestmessage when the wavelength management table is placed in the exclusivecontrol state, this ONU can receive the response message M2, because itsvariable wavelength filter 320 is set to the common wavelength λdc.However, since this ONU can discard the received message M2 that is notaddressed to it by checking the destination address 51 of the receivedmessage, there is no possibility that the same wavelengths λux, λdx areallocated to a plurality of ONUs erroneously.

FIG. 8 is a sequence diagram of a health check that the OLT 10 carriesout periodically with ONUs in operation state.

The processor 101 of the OLT 10 refers to the wavelength managementtable 120, periodically issues health check messages (410-1 to 410-x)through all INF units 12-1 to 12-x (downstream wavelengths λd1 to λdx)corresponding to the values of the wavelength index 121 whose use state122 is “under use”, and waits for response from ONUs (411). Uponreceiving the health check message, the processor of each operating ONU301-j (j=1 to k) returns a response message to the OLT 10 (412-1 to412-x).

If an INF unit 12-i was found that becomes unable to receive a responsemessage within a given period of time after the health check message wastransmitted due to, for example, the power-off of the corresponding ONUor other reason (413), the processor 101 of the OLT 10 changes the usestate 122 from “under use” to “idle” in the table entry 120-icorresponding to the INF unit 12-i in the wavelength management table120. Then, the processor 101 deactivates the INF unit 12-i (415). Whenan INF unit 12-i that has become unable to receive a response message isfound, the processor 101 may place the wavelength management table 120in the exclusive control state to change the use state in the tableentry 120-i corresponding to the INF unit 12-i, and release theexclusive control after that.

According to the foregoing embodiments, it is possible to ensure a widebandwidth because each subscriber connection apparatus ONU can transmitand receive data at its dedicated wavelength allocated from the OLT. Inthe case of increasing the number of ONUs, the engineer involved doesnot need to care for wavelengths to be used by the additional ONUbecause each ONU is able to autonomously start data transmission andreception at individual transmitting and receiving wavelengths specifiedfrom the OLT. Furthermore, since a simple optical power splitter thatdoes not need to have a wavelength separation function is applicable tothe PON, it is possible to make relatively smooth transition from aconventional PON system using the optical power splitter to a WDM-PON.

1. A WDM (Wavelength Division Multiplexing) type PON (Passive OpticalNetwork) system in which a central office side apparatus OLT (OpticalLine terminal) and a plurality of subscriber connection apparatuses ONUs(Optical Network Units) are connected by a passive optical network andeach of said ONUs communicates data with said OLT, using its specificupstream and downstream optical signal wavelengths, said passive opticalnetwork comprising an optical fiber connected to said OLT, a pluralityof branch optical fibers each connected to one of said ONUs, and asplitter for distributing to said branch optical fibers a wavelengthmultiplexed downstream optical signal transmitted from said OLT andcombining upstream optical signals transmitted from said ONUs into awavelength multiplexed signal which is transferred along the opticalfiber to said OLT, each of said ONUs comprising an optical transmittervariable its transmitting wavelength for upstream optical signals, anoptical signal receiving filter variable its receiving wavelength, and acontrol unit for controlling the transmitting wavelength of the opticaltransmitter and the receiving wavelength of the optical signal receivingfilter, said OLT comprising a wavelength management table having aplurality of entries each indicating an identifier of a wavelengthavailable in said passive optical network and a current use state of thewavelength, and a control unit for searching the wavelength managementtable for an identifier of idle state wavelength in response to awavelength allocation request from each of said ONUs and allocating atransmitting wavelength and a receiving wavelength corresponding to thewavelength identifier to the requester ONU, wherein the control unit ofeach of said ONUs transmits a wavelength allocation request to said OLTwhen the transmitting wavelength of the optical transmitter and thereceiving wavelength of the optical signal receiving filter are set atpredetermined wavelengths common for all ONUs, and the control unit ofsaid ONU switches the transmitting wavelength of the optical transmitterand the receiving wavelength of the optical signal receiving filter tospecific wavelengths allocated from said OLT in response to a wavelengthallocation response message from said OLT.
 2. The WDM type PON systemaccording to claim 1, wherein said OLT further comprises: a commoninterface unit for transmitting and receiving optical signals atwavelengths that are common for all said ONUs; a plurality of individualinterface units for transmitting and receiving optical signals atwavelengths corresponding to wavelength identifiers registered in saidwavelength management table; and an optical unit for separating awavelength multiplexed optical signal incoming from said optical fiberinto individual wavelength optical signals to selectively distribute theindividual wavelength optical signals to the interface unitscorresponding to the individual wavelengths, and wavelength multiplexingthe optical signals output from the interface units to transmit as saidwavelength multiplexed downstream optical signal to said optical fiber,wherein the control unit of said OLT communicates control messages forwavelength allocation with each of said ONUs through said commoninterface unit.
 3. The WDM type PON system according to claim 2, whereinthe control unit of said OLT activates one of said individual interfaceunits after allocating a transmitting wavelength and a receivingwavelength to one of said ONUs, said activated individual interface unittransmitting and receiving optical signals at said allocatedtransmitting wavelength and said allocated receiving wavelength.
 4. TheWDM type PON system according to claim 3, wherein the control unit ofsaid ONU having received said wavelength allocation response messagefrom said OLT transmits a wavelength allocation acknowledgement messageto said OLT, and the control unit of said OLT actives said individualinterface unit after receiving the wavelength allocation acknowledgementmessage.
 5. The WDM type PON system according to claim 3, wherein thecontrol unit of said OLT changes, in response to said wavelengthallocation acknowledgement message, the use state of wavelength toin-use state for an entry corresponding to said allocated wavelengthidentifier in said wavelength management table.
 6. The WDM type PONsystem according to claim 2, wherein the control unit of said OLTperiodically transmits control messages for checking operating state ofONUs through said individual interface units specified by theidentifiers of the wavelengths that are registered as in use in saidwavelength management table, and changes the use state to idle state inan entry of said wavelength management table, having a wavelengthidentifier corresponding to the individual interface unit that failed toreceive a response message from the corresponding ONU within a givenperiod of time.
 7. The WDM type PON system according to claim 6, whereinthe control unit of said OLT deactivates the individual interface unitthat failed to receive a response message from the corresponding ONUwithin a given period of time.
 8. The WDM type PON system according toclaims 1, wherein the control unit of each of said ONUs issues saidwavelength allocation request message after executing a predeterminedcommunication sequence for connecting to said OLT.